1. Field of the Invention
The present invention generally relates to physical vapor deposition of a metal film. More particularly, the present invention relates to high density plasma physical vapor deposition of a metal into high aspect ratio features on a substrate surface.
2. Background of the Related Art
Physical vapor deposition (PVD) or sputtering is a known technique used in the manufacture of integrated circuits. In PVD, a target of a desired coating material is bombarded by ions accelerated thereto to dislodge and eject target material from the target which is then deposited onto a substrate. The target and the substrate to be coated are generally placed in a vacuum chamber which is evacuated to and maintained at a pressure of less than about 10 milliTorr. Typically, a heavy inert gas such as argon is supplied to the vacuum chamber and a pumping system maintains the desired gas pressure in the chamber. A glow discharge plasma is created in the low pressure argon, at least partially ionizing the gas, by supplying a high negative DC, AC or RF potential to a cathode (typically the target) and grounding the chamber walls and an anode (typically the substrate). The glow discharge plasma is created in the space between the cathode and the anode, and is generally separated from the electrodes by a dark space or plasma sheath. Since the plasma itself is a good conductor, the plasma remains at essentially a constant positive potential with respect to the negatively biased cathode. This produces an electric field at the target that is substantially perpendicular to the exposed surface of the target. Thus, positive ions from the plasma are accelerated across the dark space onto the exposed surface of the target following trajectories substantially perpendicular to the exposed front surface of the target resulting in sputtering of the target.
The goal in most deposition processes is to deposit a film of uniform thickness across the surface of a substrate, while also providing good fill of lines, interconnects, contacts, vias and other features formed on the substrate. The most widely used metal deposition materials include tungsten (W), titanium (Ti), titanium nitride (TiN), aluminum (Al), and copper (Cu). As device geometries shrink, it has become increasingly difficult to deposit materials to form conformal barriers and liners, such as Ti and TiN, into these device features to form low resistance interfaces.
With recent decreases in the size of semiconductor devices and corresponding decreases in device features to less than a quarter micron (&lt;0.25 .mu.m) in aperture width, conventional S sputtering (i.e., PVD) has not been satisfactory in providing sufficient deposition thickness at the base of high aspect ratio features, i.e. those where feature depth exceeds the feature width. Furthermore, conventional PVD can cause overhang, or a crowning effect in the region near the top comers of the feature opening, and thereby restrict the path for subsequent deposition. Thus, an advanced deposition process is required to deposit metals in these smaller features having increased aspect ratios and to achieve sufficient bottom coverage.
More recently, high density plasma (HDP) processes, including ionized metal plasma (IMP), have been used to enhance deposition into these features. Generally, a HDP PVD chamber, as shown in FIG. 2, includes a target 11, a substrate support pedestal 18, a RF coil 31 disposed in the region between the target and the substrate, and a substrate power supply 48. To improve bottom coverage, HDP PVD uses a high density plasma between a sputtering target and a substrate to ionize a high fraction of sputtered atoms before depositing onto the substrate. These ions are accelerated perpendicularly towards the surface of the substrate within a plasma sheath, improving the selective or preferential filling of high aspect ratio features. Additionally, biasing of the substrate relative to plasma potential is widely used in HDP PVD to control the energy of ions reaching the substrate and improve results. The bias voltage can be applied directly to the substrate support, or to a conductive substrate or a conductive layer on the substrate through the support. With a non-conductive or highly resistive substrate, a radio frequency (RF) bias voltage can be capacitively coupled to the substrate. The radio frequency of the RF bias is typically between 10 Kilohertz and 300 Megahertz.
An example of a PVD system capable of improved deposition into small high aspect ratio features is the VECTRA.TM. IMP PVD System, available from Applied Materials, Inc., Santa Clara, Calif. The IMP PVD system improves bottom coverage and reduces the overhang formation by directing the sputtered metal particles normal to the substrate surface so that the particles can deposit into deep, sub-quarter micron features. An RF powered coil is placed between the target and the substrate to create a dense plasma (n.apprxeq.10.sup.12 cm.sup.-3) in the space between the target and the substrate. Sputtered metal atoms become ionized as they travel through this region. As the ionized metal atoms approach the plasma boundary near the substrate, the electric field caused by the applied substrate and/or the substrate support bias directs the ionized metal atoms normal to the substrate surface. Because the ionized metal atoms are traveling normal to the surface of the substrate, they can deposit into the bottom of high aspect ratio features without hitting the side walls of the features and forming overhangs. Generally, the IMP PVD system can achieve sufficient bottom coverage without forming significant overhangs.
However, an applied bias to the substrate causes re-sputtering of the deposited material at the top portion of the feature aperture. The amount of re-sputtering increases with the power of the applied bias to the substrate The re-sputtered material deposits onto the side walls of the aperture and forms overhangs. This undesirable crowning effect restricts subsequent deposition into the aperture. Because the bottom coverage and the formation of overhangs depends on the bias power applied at the surface of the substrate, HDP PVD still presents problems when a higher bottom coverage is desired. FIG. 1a is a cross sectional view of a high aspect ratio feature deposited using HDP PVD techniques at a high (.apprxeq.400 W) electrostatic chuck bias, and FIG. 1b is a cross sectional view of a high aspect ratio feature deposited using HDP PVD techniques at a low (.apprxeq.200 W) electrostatic chuck bias. Both FIGS. 1a and 1b illustrate deposition of 1000 .ANG. of titanium nitride (TiN) on the surface of the substrate. When high bias is applied, bottom coverage improves to between 35% and 46% in a high aspect ratio feature having 0.35 .mu.m width and 1.2 .mu.m depth. However, the high bias causes re-sputtering of the deposited material from the deposited material near the top edge of the feature to form large overhangs on the side walls near the upper portion of the aperture, which again restricts subsequent deposition. When low bias is applied, the overhang formation is minimized, but the bottom coverage decreases as well because a lesser amount of deposition is directed by the substrate bias to the bottom of the feature.
Therefore, there remains a need for a metal deposition process for high aspect ratio sub-quarter micron features that can achieve good bottom coverage while minimizing overhang formations near the openings of features formed on substrates. SUMMARY OF THE INVENTION
The invention generally provides a method for depositing a metal film on a substrate, comprising generating a high density plasma in a chamber, sputtering metal particles from a target to the substrate, and applying a modulated radio frequency (RF) bias to the substrate during deposition. In one aspect of the invention, the modulated RF bias comprises cycling between a first RF bias and a second RF bias. Preferably, the modulation cycle of the substrate bias is repeated during deposition of a metal film to achieve high bottom coverage and minimal overhang.
Another aspect of the invention provides an apparatus for depositing a metal film on a substrate comprising a high density plasma physical vapor deposition (HDP PVD) chamber and a controller to modulate a RF bias power applied to a substrate in the chamber.